This invention relates generally to bimetal electrodes for spark plugs, and more particularly to bimetal electrodes for spark plugs which utilize a heat resisting metal for the core and exposed tip portion and a highly thermally conductive material for an outer layer away from the tip and to novel and improved methods and apparatus for producing such electrodes.
Because of the greatly lengthened service periods for modern spark ignition engines, efforts have been made to increase the life of spark plugs, particularly to minimize the erosion of the electrodes, which tends to increase the spark gap of the plug. The greatest problem of spark plug erosion occurs at the central electrode as it projects from the insulator toward the other electrode which is welded to the shell. These central electrodes have been made from relatively pure nickel materials because of the high temperature properties of nickel and its resistance to any chemical reaction during the combustion process. It has been recognized that one of the causes for erosion is the fact that the tip of the central electrode reaches a very high temperature, in part because of the relatively poor thermal conductivity of metallic nickel. In addition to metallic nickel, efforts have been made to use certain nickel alloys, such as Inconel, which is an alloy of nickel with chromium and a small amount of iron, because of the improved temperature resistance of such alloys. However, such efforts have not produced substantially longer electrode life because such alloys tend to have an even lower coefficient of thermal conductivity than nickel itself, so that they may operate at even higher temperatures than a pure nickel electrode.
It has been recognized that a way to reduce the temperature of the center spark plug electrode is to utilize a metal having a high degree of thermal conductivity in combination with the nickel to improve heat transfer away from the firing tip. However, there are limitations in the way this may be done because of the physical structure of the spark plugs. Because the plug is exposed to high pressures as well as temperatures within the combustion chamber, it has been found necessary to ensure that the center electrode be a single unitary piece extending from the firing tip through the ceramic insulator to an external portion, where it is generally anchored in place. Thus, the physical considerations of the spark plug structure have mandated the shape of the central electrode.
It has also been recognized that the best material for providing high thermal conductivity is metallic copper of a high degree of purity, and while such a copper material may have a coefficient of thermal conductivity more than ten times that of certain nickel alloys used in the electrode structure, the copper has a much lower melting point and is chemically much more active, so that if exposed to the high temperatures and other conditions in the combustion chamber, any pure copper would rapidly erode and corrode to the point that it would be inefficient as a heat transfer medium. For this reason, it has long been proposed to provide an electrode structure in which nickel or a nickel alloy forms the exposed portion of the electrode, while copper is provided as an inner core, preferably extending as close as possible to the firing tip and extending backward continuously to the terminal end of the plug. However, there are many problems in fabricating such a bimetallic electrode, since this generally requires the making of a cup formed of the outer material into which must be assembled a copper slug and a resultant combination then extruded or drawn to the smaller diameter of the finished electrode without permitting the formation of any voids within the electrode, which would eliminate the effectiveness of the copper as a thermal conductor. Furthermore, such methods are quite expensive because they require several stages of operation on different machines, with necessary transfers between them, and the higher cost of manufacture is an undesirable detriment to the commercial use of spark plugs having such electrodes.
It has also been recognized that a center electrode could be used in which copper was an outer layer over a nickel or nickel alloy core so long as only the core was exposed at and near the firing tip. As long as the outer layer of copper is protected by the ceramic insulator and only the nickel or core material extends outward from the insulator to form the firing tip, the copper is suitably protected so that it will not erode or corrode during normal engine operation. However, since the physical construction of the spark plug requires that the electrode have a uniform diameter both within the ceramic insulator and at the exposed firing tip, the manufacture of such electrodes in a simple and low-cost process has not heretofore been possible.